1. Field of the Invention
The invention relates to electrochemical reactions and electrodes used therein. More particularly, the invention relates to electrodes for use in the electrochemical removal of nitrates from water and methods for making such electrodes.
2. State of the Art
Parent applications Ser. No. 08/671,264 and Ser. No. 08/457,040 disclose methods and apparatus for the removal of nitrates from water. Although not limited thereto, the apparatus generally includes an electrochemical flow cell through which the aqueous solution containing nitrates flows or a holding tank cell into which the solution is introduced and then released after processing, and an electrode system including a carbon fiber cathodic electrode, a carbon fiber anodic electrode and a reference electrode. All of the electrodes are immersed in the aqueous solution and coupled to an electronic control circuit which impresses a voltage across the electrodes such that the voltage causes electrochemical reduction/oxidation reactions on the surfaces of the cathodic and anodic electrodes. According to the method, the electrodes are at a potential wherein nitrates are reduced to gaseous products but hydrogen, oxygen, chlorine, and other noxious substances are not produced. According to the disclosed preferred embodiment, the reference electrode is a silver/silver-chloride electrode, the cathodic and anodic electrodes are carbon fibers based on polyacrylonitrile (PAN), and the surface area ratio of the anodic electrode to the cathodic electrode is preferably in the range of 40:1 to 120:1.
As disclosed in these parent applications, the anodic to cathodic surface area ratio must be large in order to prevent a chlorine evolution reaction from taking place in salt water and to prevent oxygen evolution reactions and changes in pH in fresh water. In practice, it has been discovered that, in seawater, an anodic to cathodic surface area ratio of up to 150:1 is desirable to prevent chlorine formation under any circumstances. In addition, the anodic voltage (relative to the reference electrode) must be kept below +800 mV to prevent chlorine formation in seawater. Moreover, in flow-through systems, an increased flow rate (flow velocity relative to the cathode) increases the nitrate reduction current and, as a consequence, causes an increase in the anodic voltage.
Parent application Ser. No. 08/758,584 discloses a carbon fiber electrode which is coated with a noble metal oxide to create a noble metal oxide electrode with a very large surface area. According to a disclosed preferred embodiment, the noble metal oxide is iridium oxide. A disclosed method of making the electrode includes preparing a solution of iridium chloride and isopropyl alcohol, dipping a carbon fiber electrode into the solution, drying the electrode in the presence of nitrogen and heat, and heat treating the electrode in the presence of oxygen. A disclosed apparatus for removing nitrates from water includes an electrochemical flow cell through which the aqueous solution containing nitrates flows or a holding tank cell into which the solution is introduced and then released after processing, and an electrode system including an anodic iridium oxide coated carbon fiber electrode as described above, a carbon fiber cathodic electrode and a reference electrode. All of the electrodes are immersed in the aqueous solution and coupled to an electronic control circuit which impresses a voltage across the electrodes such that the voltage causes electrochemical reduction/oxidation reactions on the surface of the cathodic electrode. The electrodes are at a potential wherein nitrates are reduced but hydrogen, oxygen, and chlorine are not produced.
All of the carbon fiber and noble metal oxide electrodes disclosed in the parent applications have a relatively high surface area to volume ratio. This property has the advantage of allowing the provision of a relatively compact system while maintaining the required anodic to cathodic surface area ratios. It has been discovered that the efficiency at which the cathodic electrodes operate is also related to how much of the water being processed actually comes in contact with the cathodic electrode surface. It is therefore desirable to configure and place cathodic electrodes in order to maximize the amount of water which actually impinges on the cathodic electrode surface. This is particularly important in large volume systems where a relatively large volume of water needs to be processed in a relatively short period of time.